1. Field of the Invention
The present invention relates to a semiconductor device manufacturing method and a semiconductor device, particularly to a method for assembling a semiconductor device comprising a wiring board and a semiconductor chip, mounted on the wiring board as a flip chip and thereafter secured by an underfill resin.
2. Related Art
A conventional semiconductor device in which a semiconductor chip is mounted on a printed wiring board (hereafter referred to as PWB) as a flip chip has problems in that the PWB is deformed due to the thermal expansion rate difference between the semiconductor chip made of silicon and the PWB which results in cracking of a solder joint. Therefore, the above problems have been conventionally prevented by injecting an epoxy-based underfill resin between a semiconductor chip and a PWB, and hardening the resin to reinforce the portion around a solder bump.
FIG. 1(a) is a top view showing a conventional example and FIG. 1(b) is a sectional view of the conventional example, taken along the line B-B' of FIG. 1(a). A semiconductor chip 1 is mounted on a glass-epoxy PWB 4 as a flip chip, and then covered and sealed with a sealing member constituted of a reinforcement frame 6 and a plate member 11.
That is, the semiconductor chip 1 is electrically connected to an electrode (not illustrated) on the PWB 4 through solder bumps 2 and an underfill resin 3 is injected between the semiconductor chip 1 and the PWB 4 in order to prevent the bump 2 from being damaged, and the resin is thereafter hardened through curing.
Moreover, an annular reinforcement frame 6 made of copper is previously bonded onto the PWB 4 at the outer periphery of the semiconductor chip 1 by a bonding resin 5. The annular reinforcement frame 6 has a constant separation from the semiconductor chip 1. A plate member 11 is bonded by bonding resins 10 and 9, and the semiconductor chip 1 is thereby sealed.
In the case of the above conventional packaging method, however, the PWB 4 may warp or the semiconductor chip 1 may be deformed such that a crack 17 occurs because the underfill resin 3 contracts during hardening.
The conventional packaging method is now described in detail.
FIGS. 2A to 2F are sectional views showing a conventional semiconductor-device fabrication process for the structure of FIG. 1.
First, in FIG. 2A, a plurality of pads 12 are previously arranged on one side of a rectangular PWB 4 and external electrodes 7 are arranged on the other side of the PWB 4. Moreover, though not illustrated, each of the pads 12 and the external electrodes 7 are electrically connected through a wiring layer in the PWB 4. Moreover, a reinforcement frame 6 for fixing a plate member 11 is previously bonded to the periphery of the PWB 4 by a bonding resin 5. The reinforcement frame 6 delimits a space for setting a semiconductor chip 1 between the PWB 4 and the plate member 11.
In FIG. 2B, each solder bump 2 and each pad 12 corresponding to a respective bump 2 are aligned and thereafter, the semiconductor chip 1 is mounted on the PWB 4.
In FIG. 2C, by melting the bump 2 and connecting it with the pad 12, the semiconductor chip 1 is mounted on the PWB 4. This is generally referred to as flip-chip mounting.
In FIG. 2D, to prevent the bump 2 from becoming disconnected, a flowable underfill resin 3 is injected into the gap between the semiconductor chip 1 and the PWB 4. In this case, because the gap between the semiconductor chip 1 and the PWB 4 is very small, the injected underfill resin 3 expands over the entire surface of the semiconductor chip 1 due to the capillary phenomenon.
In FIG. 2E, the injected underfill resin 3 is hardened by curing (heat-treating). Then, because the epoxy-based underfill resin 3 slightly decreases in volume due to hardening, the PWB 4 is deformed so that the central portion of it warps upward.
Thereafter, in FIG. 2F, bonding resins 10 and 9 are applied to the "back" of the semiconductor chip 1 (side opposite to the active-element forming side) and the upper side of the reinforcement frame 6 and then, the plate member 11 is mounted so as to cover the semiconductor chip 1 and the reinforcement frame 6. Then, by curing the bonding resins 10 and 9 again to harden them, the plate member 11 is fixed and a package is completed.
As shown in FIG. 3, however, these components are different from each other in thermal expansion coefficient and elastic modulus. Particularly, because the thermal expansion coefficient of the underfill resin 3 is greatly different from those of the semiconductor chip 1 and the PWB 4, the PWB 4 and the semiconductor chip 1 are warped or distorted when the underfill resin 3 is hardened. However, when a package having a distorted PWB 4 and semiconductor chip 1 is mounted on a mounting board, it is impossible to accurately connect the bump 8 serving as an external electrode to a pad on the mounting board. Therefore, there is a problem that imperfect mounting readily occurs.
Moreover, as shown in FIG. 2F, when the bonding resin 10 applied to the back of the distorted semiconductor chip 1 is hardened, there is also a problem in that a crack 17 occurs in the semiconductor chip 1 due to a stress produced under hardening.